1. Field of the Invention
The present invention relates to a substrate (base plate) for mass spectrometry and a mass spectrometry method. In the mass spectrometry, a substance fixed (immobilized) on a surface of the substrate is irradiated with a laser beam to be desorbed from the surface, and the desorbed substance is captured to perform mass spectrometry.
2. Description of the Related Art
As an analysis method used to identify a substance or the like, a mass spectrometry method is well known. In the mass spectrometry method, an analyte (a substance to be analyzed, an analysis target) is ionized, and identified based on the mass-to-charge ratio of the analyte. For example, in time-of-flight mass spectrometry (TOF-MS, time-of-flight mass spectroscopy), an ionized analyte is caused to fly for a predetermined distance between high-voltage electrodes, and the mass of the analyte is analyzed based on the time of flight.
As ionization methods that are used in the mass spectrometry, there are an electric-field desorption method (FD), a fast atom bombardment method (FAB), a matrix-assisted laser desorption ionization method (MALDI), an electrospray ionization method (ESI), and the like. However, since some analytes are easily ionized while some other analytes are not easily ionized, if ionization process is performed on a mixture of such analytes to ionize the analytes that have different ionization characteristics from each other at the same time, the sensitivity of detecting the analyte that is not easily ionized is lower than the sensitivity of detecting the analyte that is easily ionized. Consequently, a problem of non-uniform detection occurs. Therefore, LC-ESI, in which a liquid chromatography (LC) and the ESI are combined, is generally used as a method for performing mass spectrometry after a plurality of analytes are separated from each other. However, in the LC-ESI, only an analyte that has passed through an LC column is ionized. Therefore,  an analysis time period is long, and loss of the analyte occurs due to adsorption of the analyte to the column. To solve such problems, U.S. Patent Application Publication No. 20060214101 and “Desorption/Ionization on Silicon Nanowires”, E. P. Go et al., Analytical Chemistry, Vol. 77, pp. 1641-1646, 2005 propose methods for performing mass spectrometry without loss and in a short time period by separating the analytes and by directly irradiating the separation area with a laser beam.
However, in these methods for mass spectrometry, a high-power laser beam is necessary to ionize a substance adsorbed on the surface of a substrate and to desorb the substance from the surface of the substrate. If the high-power laser beam is used, there is a risk that the analyte is damaged. Further, since a high-power light source is needed to irradiate the analyte with the high-power laser beam, there is a problem that the cost of the apparatus becomes high. Further, as a method for desorbing the analyte by using a weak laser beam, a method of spraying a matrix material onto the surface of the substrate after separating the analytes has been proposed. However, in the method, the analytes are spread and blurred by application of the matrix material. Therefore, the separation condition of the analytes deteriorates.
U.S. Pat. No. 7,579,588 proposes an analysis method and apparatus using a substrate for mass spectrometry that includes a rough metal surface on the surface of the substrate. The rough metal surface can generate localized plasmons, and avoid irradiation with a high-power laser beam to desorb the analyte from the substrate. In this method, the low-power laser beam can desorb/ionize the analytes without damaging the analytes. However, separation of the analytes, as described above, is impossible.
Further, U.S. Pat. No. 7,586,091 proposes a method for improving the ionization efficiency by plasmons. In this method, a microchip (substrate) including a plurality of columnar members in a sample separation area of the microchip is used. Further, a metal layer is provided on the surfaces of the columnar members. However, improvement of the ionization efficiency by plasmons is  neither described in detail nor sufficiently studied.